The invention is based on a priority application EP 01 440 309.1 which is hereby incorporated by reference. The present invention relates to a multicarrier receiver suitable to receive a sequence of cyclically extended multicarrier symbols.
DSL (Digital subscriber Line) enables high speed digital data transport over telephone lines. In some applications such as ADSL (Asymmetric Digital Subscriber Line), this is done in overlay on the analogue POTS (Plain Old Telephone Service) service. Thanks to ADSL, telephone companies can reuse most of their installed wiring for the introduction of new services. By using DMT (Discrete Multi Tone) modulation carriers with a higher signal to noise ratio (SNR) are allowed to carry more bits than carriers with a low SNR.
The effect of intersymbol and intercarrier interference due to transmission of the DMT symbols over a channel between multicarrier transmitter and multicarrier receiver can be removed by adding a cyclic extension (CE) to each DMT symbol with a length superior to the channel impulse response length. The data rate, however, reduces proportionally to the length of the cyclic prefix that is added to the DMT symbols so that the length of the cyclic extension of DMT symbols has to be limited to an acceptable number. If the channel impulse response is larger than the cyclic extension, remaining intersymbol interference (ISI) will depend on the part of the impulse response exceeding the cyclic extension length.
In order to shorten the channel's impulse response, the use of a time domain equalizer has been suggested. FIG. 6 shows a known multicarrier receiver RX′ which is able to receive and demodulate multicarrier symbols MS that are cyclically extended. It is the task of the time domain equalizer TEQ to shorten the impulse response length of the channel so that it does not exceed the length of the cyclic extension CE. The time domain equalizer TEQ thereto contains a set of adaptive taps whose values are set in accordance with a mean square error (MSE) criterion. If a multicarrier symbol MS has passed the equalized channel (transmission channel+time domain equalizer TEQ), the samples thereof are serial to parallel converted by the serial to parallel converter S/P′ and the cyclic prefix extractor CE EXTRACT subtracts the cyclic extension CE from the multicarrier symbol MS so that a non-extended multicarrier symbol is applied to a Discrete Fourier Transformer (DFT), efficiently implemented as the fast Fourier transformer FFT′ for time to frequency domain conversion. In the remainder of the description, the terms ‘DFT’ and ‘FFT’ will both be used. It is important to keep in mind that the FFT algorithm is merely an efficient manner to calculate a DFT, but both have the same effect. Obviously, any practical implementation will make use of the FFT. The fast fourier transformer FFT′ may be preceded by a time domain window WIN′, to decrease spectral leakage. The frequency domain multicarrier symbol at the output of the fast Fourier transformer FFT′ is supplied to the frequency domain equalizer FEQ′ which typically contains one complex tap per carrier to compensate for each carrier the phase rotation and attenuation due to transmission over the channel. For the so obtained carriers the demapper DMAP′ decodes the exact amount of bits from each carrier using the appropriate constellation schemes. The bits at the output of the demapper DMAP′ are serialised by the parallel to serial converter P/S′.
The time domain equalizer TEQ in a multicarrier receiver RX′ with the known architecture cannot treat different carriers of the multicarrier symbol MS differently although different carriers may be differently affected by noise on the transmission channel. An improved multicarrier receiver RX″ known from EP 969 637 A1 is shown in FIG. 7. The samples of the cyclically extended multicarrier symbol MS, when received by the multicarrier receiver RX″, are paralleled by the serial to parallel converter S/P″ without having passed any equalizer. The extended multicarrier symbol MS then is supplied to the sliding fast Fourier transformer SLIDING FFT which converts different parts of the extended multicarrier symbol MS from time domain to frequency domain by calculating several consecutive Fourier transformations.
A complete calculation of these FFTs would be very computationally intensive. However, a sliding DFT (be it implemented by the FFT algorithm) can be derived from one DFT (FFT) and difference terms. Therefore, in practice the FFTs are replaced by one full FFT and difference terms, without sacrificing performance. The difference terms are formed as differences between incoming samples that are a distance equal to the FFT size apart.
The parts of the extended multicarrier symbol MS that are transformed (by the FFTs) all have the length of a non extended multicarrier symbol, i.e. the Fast Fourier Transform (FFT) size. The sliding fast Fourier transformer SLIDING FFT in this way calculates at most an amount of Fourier transforms equal to the number taps of the tapped delay lines TD1′, TD2′, . . . , TDN′/2 in the per-carrier frequency domain equalizer PC-FEQ′, which is also often called per-tone equalizer. The resulting frequency domain multicarrier symbols are applied to the per-carrier frequency domain equalizer PC-FEQ′. In the per-carrier frequency domain equalizer PC-FEQ′, each carrier is equalized by an individual equalizer or tapped delay line TD1′, TD2′, . . . , TDN′/2. The number of complex taps per tapped delay line TD1′, TD2′, . . . , TDN′/2 does not necessarily have to be the same for each tapped delay line TD1′, TD2′, . . . , TDN′/2, but could be a maximum of T taps per line. The equalized carriers at the output of the per-carrier frequency domain equalizer PC-FEQ′ are applied to the demapper DMAP″ which decodes the exact amount of bits from each carrier using the appropriate constellation schemes and the bits sourced by this demapper DMAP″ are serialised by the parallel to serial converter P/S″.
It is known that part of the cyclic extension can be used for windowing in the receiver, such as in, but not limited to a Very High Speed Digital Subscriber Line (VDSL) modem. This operation is transparent for tones that are perfectly periodic in the DFT window, but reduces the effect of transitions that would otherwise cause intersymbol and intercarrier interference. It thus helps to reduce the spectral leakage effects due to the bad spectral containment of the DFT operation. Hence, Radio Frequency Interference (RFI) and crosstalk will only affect a limited number of carriers. In order to combine the benefits of the per-tone equalization and windowing, an implementation of both in a multicarrier receiver is desirable. However the implementation of windowing and per-tone equalization would computationally be very demanding. A straightforward implementation would require the sliding DFT to be replaced by a number of successive window and DFT operations, implemented as FFTs, as shown in FIG. 8. However, the simple implementation with low computational effort (making use of the difference terms, as with the PC-FEQ′) to anticipate the per tone equalization would be lost.